The present invention relates to a cryogenic refrigeration system for cooling a superconducting magnet.
A cryogenic refrigeration system capable of generating a cryogenic temperature of a 4 K level is used as a system for cooling a superconducting magnet. For a cryogenic refrigeration system of this type, it is of course important to efficiently generate a cryogenic level of coldness, and it is also important to prevent heat from entering the system from outside. In view of this, a heat shield technique has been used in the prior art, in which part or whole of a cryogenic section of the system is covered by a low-temperature plate or cylinder member in order to prevent heat from entering the cryogenic section.
For example, Japanese Laid-Open Patent Publication No. 9-229503 discloses a cryogenic refrigeration system using a heat shield technique. The cryogenic refrigeration system includes a JT refrigerator for generating a liquid helium of a 4 K level, a helium tank for storing the generated liquid helium, a heat shield plate covering the helium tank, and a shield refrigerator for cooling the heat shield plate. Note that in the cryogenic refrigeration system, a superconducting magnet is immersed in the liquid helium in the helium tank and is cooled to a temperature that is less than or equal to the critical temperature.
The cryogenic refrigeration system employs, as the shield refrigerator, a GM refrigerator that uses a helium refrigerant, so that the JT refrigerator and the shield refrigerator can share a compressor. Specifically, the cryogenic refrigeration system includes a low pressure side compressor and a high pressure side compressor, and the JT refrigerator is supplied with a helium gas, which has undergone a two-stage compression through these compressors, whereas the shield refrigerator is supplied with a helium gas, which has been compressed only through the high pressure side compressor.
The refrigeration load of the JT refrigerator and the refrigeration load of the shield refrigerator significantly vary depending on the operating environment under which the system is used. Specifically, heat is prevented by the shield refrigerator from entering the JT refrigerator from outside, whereby the JT refrigerator is less influenced by the atmospheric temperature. However, depending on the type of operation, the refrigeration load thereof is increased by frictional heat due to mechanical vibrations and by a Joule loss due to a magnetic field. Thus, the refrigeration load may fluctuate significantly by, for example, switching between different operations. In the shield refrigerator, in contrast, the majority of the refrigeration load is due to heat entering from outside, whereby the shield refrigerator is more influenced by the atmospheric temperature while the refrigeration load thereof does not significantly fluctuate due to internal frictional heat, etc.
Typically, the capacity of a refrigerator is determined according to the maximum refrigeration load expected. Therefore, the capacity of a JT refrigerator is determined according to the maximum refrigeration load in view of the internal frictional heat, etc. However, the refrigeration load may fluctuate significantly depending on the operating conditions, as described above. Therefore, if the capacity of the JT refrigerator is fixed irrespective of the operating conditions, the refrigeration capacity may be excessive under operating conditions where the internal frictional heat, or the like, does not occur. As a result, the JT refrigerator generates an amount of liquid helium that is more than required, thereby lowering the efficiency of the system.
The problem may be addressed by a capacity control in view of the fluctuation of the refrigeration load of the JT refrigerator in order to improve the efficiency of the system, i.e., a capacity control in which the capacities of the low pressure side compressor and the high pressure side compressor are reduced when the refrigeration load due to frictional heat, etc., is small. With such a control, however, not only the refrigeration capacity of the JT refrigerator, but also the refrigeration capacity of the shield refrigerator, is reduced. Then, the refrigeration capacity of the shield refrigerator may be insufficient because the refrigeration load of the shield refrigerator is substantially constant irrespective of the operating conditions. Therefore, a new technique that can solve the problem has been longed for in the art.
The present invention has been made in view of the above, and has an object to provide a cryogenic refrigeration system capable of operating efficiently while accommodating the fluctuation of the refrigeration load.
According to the present invention, when the refrigeration load of the helium refrigerator is small, the operation of the pre-cooling circuit of the helium refrigerator is stopped while the operation of the nitrogen refrigerator is continued, thus achieving the object set forth above.
A first cryogenic refrigeration system of the present invention is a cryogenic refrigeration system for cooling a superconducting magnet, including: a helium refrigerator including a first compressor for compressing a helium gas, a second compressor provided on a discharge side of the first compressor, a JT circuit for liquefying, through a Joule. Thomson expansion, the helium gas, which has undergone a two-stage compression through the first compressor and the second compressor, and a pre-cooling circuit for pre-cooling the helium gas of the JT circuit by expanding the helium gas, which has undergone the two-stage compression; a helium tank for storing the liquid helium liquefied by the helium refrigerator and for supplying the liquid helium to the superconducting magnet; a heat shield member for preventing heat from entering the superconducting magnet by using a liquid nitrogen; a nitrogen tank for storing the liquid nitrogen and for supplying the liquid nitrogen to the heat shield member; and a nitrogen refrigerator for generating coldness by expanding the helium gas discharged from the second compressor and for cooling the nitrogen in the nitrogen tank by using the coldness. The first cryogenic refrigeration system further includes a controller for selectively performing one of a normal operation, in which the first compressor and the second compressor are operated and both of the helium refrigerator and the nitrogen refrigerator are operated, and a small-capacity operation, in which the first compressor and the second compressor are operated and an operation of the pre-cooling circuit of the helium refrigerator is stopped while the nitrogen refrigerator is operated.
A second cryogenic refrigeration system is similar to the first cryogenic refrigeration system, wherein: the second compressor is a compressor whose capacity can be controlled; and the controller controls the capacity of the second compressor so that a refrigeration capacity of the nitrogen refrigerator during the small-capacity operation is substantially the same as that during the normal operation.
A third cryogenic refrigeration system is similar to the first cryogenic refrigeration system, wherein the controller switches from the normal operation to the small-capacity operation when an amount of liquid helium in the helium tank increases above a predetermined amount during the normal operation.
A fourth cryogenic refrigeration system is similar to the first cryogenic refrigeration system, wherein the controller switches from the small-capacity operation to the normal operation when an amount of liquid helium in the helium tank decreases below a predetermined amount during the small-capacity operation.
A fifth cryogenic refrigeration system is similar to the first cryogenic refrigeration system, further including a liquid level sensor provided in the helium tank, wherein the controller switches from the normal operation to the small-capacity operation when a liquid level of the liquid helium in the helium tank rises above a predetermined position during the normal operation, whereas the controller switches from the small, capacity operation to the normal operation when the liquid level of the liquid helium in the helium tank lowers below a predetermined position during the small-capacity operation.
Note that the predetermined position based on which the operation is switched from the normal operation to the small-capacity operation may be the same as, or different from, the predetermined position based on which the operation is switched from the small-capacity operation to the normal operation.
A sixth cryogenic refrigeration system is similar to the first cryogenic refrigeration system, further including: a buffer tank connected to the JT circuit for collecting the helium gas from the JT circuit when a helium gas pressure on a high pressure side of the JT circuit increases above a predetermined upper limit value, while supplying the helium gas to the JT circuit when a helium gas pressure on a low pressure side of the JT circuit decreases below a predetermined lower limit value; and a pressure sensor for detecting a pressure of the helium gas in the buffer tank, wherein the controller switches from the normal operation to the small-capacity operation when the pressure of the helium gas in the buffer tank decreases below a predetermined pressure during the normal operation, whereas the controller switches from the small-capacity operation to the normal operation when the pressure of the helium gas in the buffer tank increases above a predetermined pressure during the small-capacity operation.
Note that the predetermined pressure based on which the operation is switched from the normal operation to the small-capacity operation may be the same as, or different from, the predetermined pressure based on which the operation is switched from the small-capacity operation to the normal operation.
With the first cryogenic refrigeration system, when the refrigeration load of the helium refrigerator is large, a refrigeration operation is performed in both of the helium refrigerator and the nitrogen refrigerator (xe2x80x9cnormal operationxe2x80x9d). On the other hand, when the refrigeration load of only the helium refrigerator decreases, the operation of the pre-cooling circuit of the helium refrigerator is stopped while the refrigeration operation of the nitrogen refrigerator is continued (xe2x80x9csmall-capacity operationxe2x80x9d). Therefore, it is possible to suppress the refrigeration capacity of the whole system without lowering the power of the nitrogen refrigerator, thereby improving the operation efficiency and reducing the power consumption.
With the second cryogenic refrigeration system, the second compressor is a compressor whose capacity can be controlled, and the capacity of the second compressor is controlled so that the refrigeration capacity of the nitrogen refrigerator during the normal operation is substantially the same as that during the small-capacity operation, whereby helium is not excessively supplied to the nitrogen refrigerator during the small-capacity operation, thus preventing the power of the nitrogen refrigerator from being excessive. In this way, it is possible to suppress the fluctuation of the power of the nitrogen refrigerator due to the operation switching, thus preventing the operation efficiency of the nitrogen refrigerator from lowering.
With the third cryogenic refrigeration system, when the amount of liquid helium in the helium tank increases above a predetermined amount during the normal operation, it is assumed that the power of the helium refrigerator is excessive, and thus the operation is switched from the normal operation to the small-capacity operation. As a result, it is possible to prevent the system from operating with an excessive power, thereby improving the operation efficiency and reducing the power consumption.
With the fourth cryogenic refrigeration system, when the amount of liquid helium in the helium tank decreases below a predetermined amount during the small-capacity operation, it is assumed that more liquid helium is required for cooling the superconducting magnet, and thus the operation is switched from the small-capacity operation to the normal operation. As a result, the pre-cooling circuit of the helium refrigerator resumes its operation, whereby the amount of liquid helium in the helium tank increases. Thus, the superconducting magnet is stably cooled to a predetermined temperature level.
With the fifth cryogenic refrigeration system, the position of the liquid level of the liquid helium in the helium tank is detected by the liquid level sensor, and the amount of liquid helium is estimated based on the position of the liquid level. When the liquid level rises above a predetermined position during the normal operation, it is assumed that the refrigeration capacity of the helium refrigerator is excessive, and thus the operation is switched from the normal operation to the small-capacity operation. When the liquid level lowers below a predetermined position during the small-capacity operation, it is assumed that the amount of liquid helium is insufficient, and thus the operation is switched from the small-capacity operation to the normal operation.
With the sixth cryogenic refrigeration system, the amount of liquid helium in the helium tank is estimated based on the internal pressure of the buffer tank provided in the helium JT circuit. When the internal pressure of the buffer tank decreases below a predetermined pressure during the normal operation, it is assumed that a sufficient amount of helium, having been stored in the buffer tank, has moved to the helium tank and is now stored in the helium tank in the form of liquid helium, and thus the operation is switched from the normal operation to the small-capacity operation. When the internal pressure of the buffer tank increases above a predetermined pressure during the small-capacity operation, it is assumed that a significant amount of helium, having been stored in the helium tank, has evaporated and is now stored in the buffer tank, and thus the operation is switched from the small-capacity operation to the normal operation.
According to the present invention, a refrigeration operation is performed in both of the helium refrigerator and the nitrogen refrigerator when the refrigeration load is large, whereas the operation of the pre-cooling circuit of the helium refrigerator is stopped while the operation of the nitrogen refrigerator is continued when the refrigeration load of the helium refrigerator decreases. Therefore, it is possible to improve the operation efficiency and to reduce the power consumption while ensuring a required level of refrigeration capacity.
By controlling the capacity of the second compressor so that the refrigeration capacity of the nitrogen refrigerator during the small-capacity operation is substantially the same as that during the normal operation, it is possible to suppress the fluctuation of the power of the nitrogen refrigerator due to the operation switching, thus improving the operation efficiency.